The present invention relates to buoys which can be dropped from an aircraft and more specifically to sound radio buoys which, when in the water, serve to provide a radio transmission of various underwater data and parameters detected by acoustic receivers. These radio buoys generally incorporate a float provided with an antenna as well as a hydrophone which, attached to the float descends to a certain depth to detect acoustic noise. The electrical signals from the hydrophone are processed and apply to the radio antenna.
The fall of the buoy after jettisoning must be decelerated and must be aerodynamically stable, so that after impact with the water buoyancy is ensured and the radio and acoustic systems are able to operate correctly.
It is known to use a balloon as a float, said balloon inflating during the fall and when it has reached the water the air is trapped in the balloon and ensures the buoyancy of the buoy.
Prior to the fall the balloon is contained within a casing, which must be detached at the start of the fall. Moreover, the lower part of the device contains a container in which there is the hydrophone with a cable drum. This cable serves to carry the hydrophone once submerged and to lead the electrical signals to the transmission antenna. After the buoy has reached the water the hydrophone and its cable must be detached from the float to descend to a depth determined by the length of the cable which has completely unwound.
A copending application of the present Applicant, Ser. No. 57422 filed July, 13 1979 describes a jettisonable airborne buoy construction incorporating a balloon which inflates during the fall, thus ensuring the buoyancy of the buoy on the water. The balloon inflates during the fall as a result of a forced entry of air into the bottom of the balloon, whilst within the balloon there is a membrane made from a flexible material which is able to seal the air inlet or outlet holes of the balloon, so that the pressure of the balloon is equal to the maximum dynamic pressure of air during the fall.
Articulated inwardly curved mechanical members called "scoops" permit the forced entry of air and the upper part of the balloon is provided with pockets, traversed by the air during the fall and these currents of air produce a pressure which stabilises the balloon in rotation during the said fall.
According to the prior art the casing is not fixed to the device and is detached at the start of the fall, which does not ensure a reliable operation, because said casing can cause damage. Furthermore in the prior art after impact on the water a trapdoor opens in the lower part of the buoy permitting the fall of the hydrophone with its cable. The trapdoor is opened by hydrostatic pressure. After release the cable drum and the hydrophone fall directly or are protected in a covering.
Descent is slow and therefore a considerable time elapses before the buoy is operational. Descent is faster if the drum is streamlined, but the volume available for the cable is reduced, which reduces the depth which can be attained by the hydrophone.